ff-matrix: Callout features

[cscheid]

Progress towards #9832.

• html

Fixes

This fixes the inconsistent border radii for callout headers in HTML, from

to

[gordonwoodhull]

I would do most of these as cases within a feature directory, instead of calling each case its own feature. They are mostly of interest to developers so they could be shown under the detailed view.

(But all questions of feature granularity are subjective.)

[cscheid]

I would do most of these as cases within a feature directory, instead of calling each case its own feature.

The question, I guess, hinges on whether the single document still allows tests and minimal examples without a clunky .qmd full of extraneous divs (for the selectors)

[gordonwoodhull]

Still separate qmd files, just in a directory. Like all these cases for CSS border, which we wouldn't want to expose to the end user in the ff matrix, but which are helpful to the developer:

https://github.com/quarto-dev/quarto-cli/tree/main/dev-docs/feature-format-matrix/qmd-files/css-properties/border

(Here I think I went too far, and some of the invalid/error cases belong in regular smoke tests.)